Optical connectors, particularly those suitable for underwater use, have undergone transformation due to development of a new type of optical element. This has been properly identified as the graded refractive index rod lens (GRIN). Characteristically the GRIN lenses are many times larger in diameter than typical fibers and have a pattern of refractive index versus radius that is cylindrically symmetrical. A striking characteristic of the GRIN lenses is their capacity for deflecting, or rather refracting, an optical ray which diverges from the lens center axis back toward the axis. The rays which leave the axis within a particular angle will eventually become parallel to the axis and particularly in the case of a 1/4 pitch GRIN lens will become collimated at the opposite end of the lens. In addition, 1/4 nominal pitch length GRIN lenses re suitable for realizing low insertion loss fiberoptic connectors by virtue of their properly of expanding and collimating the light beam profile emanating from the core of an optical fiber. GRIN lenses are increasingly popular and may be of the type of focusing glass device manufactured by the Nipon Sheet Glass Company and Nipon Electric Company, Ltd., Osaka, Japan under the Trademark SELFOC. The SELFOC GRIN lenses are versatile and are the lenses used in a variety of applications in the first of referenced pending United States patent application listed above.
An optical connector using GRIN lenses has been found to be less sensitive to alignment tolerances which are usually attributed to imperfections in the mechanism used to align the lenses and their fibers and to discrepancies caused in machining the connector body itself. Because the fibers are usually quite small, in the neighborhood of 50 microns, their light transmission capability is limited so that insertion loss attributed to misalignment can seriously impair a system's usefulness because of excessive losses. The imperfections of many of the well known coupling schemes become apparent and the art is replete with designs which seek to avoid excessive insertion losses. Obviously the combination of the advancing state-of-the-art to include the GRIN lenses can improve the optical fiber-connector interface.
The second referenced United States patent application above brings GRIN lenses and optical fibers together through a pressurized hull. The connector reduces optical insertion losses attributed to the optical fiber drawing imperfections and consequent core eccentricity within the optical fiber cladding. The referenced application locates the core of the optical fibers on the surface of the rod lens when the connector is fabricated and installed to help provide for optimum beam collimation and avoids some of the mechanisms otherwise required to physically index to the outside of the fiber cladding. Quite acceptable optical fiber-connector interfaces are produced in the laboratory and under conditions where external controls may be rigidly applied. The measurement standards to indicate an acceptable coupling are applied during an adjustment procedure and produces connectors that can only be manufactured in matched pairs, that is to say one optical fiber-connector interface and another optical fiber-connector interface must be paired to assure an acceptable light transmission between the two fibers. The reason for this is that the location of the "receiving" fiber on the face of its corresponding rod lens must be adjusted in order to maximize the light capture from the "transmitting" fiber that is installed on the matching, complementary connector. A laboratory may create a standard and match other connectors to it. All subsequent connectors are aligned to this standard. This standard however may be far short of an optimum coupling and its reproducibility i.e. making sub masters may not be easily accomplished.
Another problem is that the matched pairs of optical fiber-connector interfaces must be properly aligned with one another in order to function properly and any rotation of either optical fiber-connector interfaces can further alter the transmissivity properties of the coupling.
The last cited patent application referred to above provides for an improved coupling between a fiber and a GRIN lens and is secured by the provision of a precision bored bushing and precision machined ferrule which is inserted in the bushing. Calibrating an optically aligned coupling calls for an insertion of a right cylinder having a diameter to allow a snug fitting in the precision-bored bushing so that a calibration autocollimation can occur with an adjustable axially exposed mirrored surface that reflects a laser beam. After the cylinder has been removed the laser beam is projected through and in parallel with the axis of the precision-bored bushing. The adjustable mirror surface separated from and perpendicular to an axial projection of the axis of the precision-bored bushing reflects the projected beam back over its own path to autocollimate the projected and reflected beams. A GRIN lens is inserted into a close fitting precision-bored ferrule and an optical fiber is excited by a light source. Positioning the excited optical fiber on an axial exposed surface of the GRIN lens and monitoring the light intensity reflected from the mirror through the GRIN lens and into the optical fiber allows a securing of the optical fiber at a location on the axial exposed surface where the magnitude of the monitored light is maximum. The foregoing method of the first referenced pending patent application helps assure a more acceptable coupling between a fiber and lens in a coupler.
Thus there is a continuing need for a readily reproducible alignment and coupling between an optical fiber and a GRIN lens in a connector that is readily accomplished and has the feature of being interchangeable.